A private branch exchange (PBX) is a privately owned telephone switch that permits users to call one another on private telephone lines, and to share a certain number of outside lines for making telephone calls outside the PBX. Wireless enabled PBX systems provide significantly advanced mobility for users of the PBX system, since they permit PBX users to carry their telephones away from fixed locations, such as desks and/or offices. A wireless enabled PBX generally has the ability to communicate with mobile telephones through a number of microcell or picocell antennas located within or near a premises served by the PBX.
While some standards exist for private wireless telephone systems, the wireless communications protocols used in wireless enabled PBX systems need not be standardized, since wireless enabled PBX systems generally serve private entities, such as a business. In particular, wireless enabled PBX systems may use access methods that are the same as or different from those such as the FDMA (Frequency Division Multiple Access), TDMA (Time Division Multiple Access), TDD (Time Division Duplex), and CDMA (Code Division Multiple Access) access methods commonly used in standardized public wireless telephone networks. For example, a private wireless enabled PBX system may in some cases operate according to any of the above air interface standards, the DECT (Digital European Cordless Telephone) standard, or a non-standard interface. In some cases, a wireless enabled PBX may communicate with mobile telephones through a WiFi connection, such as an 802.11(b) or (g) wireless connection, which provides multiple access to the air interface using a carrier sense multiple access with collision avoidance (CSMA/CA) scheme.
An IP-PBX is a PBX that converges voice and data networks. Calls may be switched between VoIP (voice over Internet Protocol or IP) users on local lines while allowing all users to share a certain number of external phone lines. A typical IP-PBX can also switch calls between a VoIP user and a traditional telephone user, or between two traditional telephone users in the same way that a conventional PBX does. In an IP-PBX, Internet access, as well as VoIP communications and traditional telephone communications, may be provided to each user on a single line, which may provide flexibility, and may also reduce long-term operation and maintenance costs. Thus, a wireless enabled PBX may be implemented as a wireless IP-PBX which provides VoIP communications with wireless terminals using an 802.11(b) or (g) interface.
Despite the convenience and added mobility offered by a wireless enabled PBX system, such a system may be less desirable if a user's mobile telephone communicates only with the wireless enabled PBX system. In particular, it may be beneficial for a user to be able to take his or her mobile telephone along when leaving the area served by the wireless enabled PBX system, and use the mobile telephone on another network, such as a FDMA, TDMA, TDD, and/or CDMA based public wireless telephone network. The ability to use the same phone on both a private wireless enabled PBX system and a public wireless communications system may free a user from having to keep track of separate mobile telephones for use in the wireless enabled PBX system on the one hand and the public wireless telephone network on the other hand.
In order to permit a mobile telephone to be used with both a private wireless enabled PBX system and a public wireless telephone system, mobile telephones may be provided as dual mode devices capable of communicating over multiple frequency ranges using multiple air interface protocols. That is, in one mode (Public Mode), a mobile telephone may be configured to communicate in a public (licensed) frequency band over a cellular/PCS network using a standard wireless telephone communication air interface, such as IS-54, IS-95, GSM, etc. In another mode (Private Mode), a mobile telephone may be configured to communicate with a particular private wireless communication network, such as a wireless enabled PBX system, when it is in range of the private system. A wireless enabled PBX system may, but need not, use a WiFi interface similar to the WiFi interface designed for data network communications for communications with a mobile telephone.
When a mobile telephone is within range of the wireless enabled PBX system, for example, when the user of the mobile telephone is within a building/campus/area served by the wireless enabled PBX system, the mobile telephone may preferentially connect to the wireless enabled PBX system. Calls to/from the mobile telephone are managed by the wireless enabled PBX, which may in some cases be an IP-PBX.
In conventional systems, when the mobile telephone moves out of range of the wireless enabled PBX system while a call is in progress (for example if the user leaves the building/campus/area that is served by the wireless enabled PBX system), the wireless enabled PBX may be configured to handover the existing call to a public wireless network to which the user is a subscriber.
In order to execute the handover, conventional PBX systems are configured to set up a conference bridge between the existing call and the subscribers' mobile telephone number. Once the conference bridge has been set up, the connection between the wireless enabled PBX and the mobile telephone is dropped, and the call continues to be carried by the public wireless network carrier.
If the user receives a call at the PBX, the PBX may be configured to forward the call to the public wireless network if it is determined that the user is not active on the wireless enabled PBX system. Using conventional call processing techniques, if the user's mobile telephone is not active on the wireless carrier's communications system, or if the user fails to answer the call, the caller will be transferred to the voicemail system of the public wireless communications system. This may confuse the caller, since the caller initially called the user at the user's telephone number at the wireless enabled PBX, and would naturally expect that if the user is not available, the call would be answered by the PBX voicemail system or an attendant at the PBX location. Instead, the caller would be greeted with the voicemail greeting associated with the user's public wireless network carrier.
For example, if a caller called a user at the user's place of work, the caller would naturally expect that if the user is unavailable, the call would be transferred to the user's work voicemail system and/or would be given the option to speak with an attendant or some other person at the user's place of work. If instead the caller were transferred to a different voicemail system, such as the voicemail system of the user's public wireless telephone system, the caller may be presented with an unexpected voicemail greeting, and may be confused or frustrated at not being able to reach someone at their place of work.